The present invention relates to a proton-conducting polymer electrolyte membrane based on a polyazole salt of an inorganic or organic acid which is doped with an acid as electrolyte, a process for producing the proton-conducting polymer electrolyte membrane, a membrane-electrode assembly comprising the proton-conducting polymer electrolyte membrane and a fuel cell comprising the membrane-electrode assembly of the invention.
Proton-conducting, i.e. acid-doped, polyazole membranes for use in PEM fuel cells are known from the prior art. The basic polyazole films are generally doped with concentrated phosphoric acid or sulfuric acid and then act as proton conductors and separators in polymer electrolyte membrane fuel cells (PEM fuel cells). Due to the excellent properties of the polyazole polymer, such polymer electrolyte membranes can, when processed to produce membrane-electrode assemblies (MEAs), be used in fuel cells at long-term operating temperatures above 100° C., in particular above 120° C. This high long-term operating temperature allows the activity of the noble metal-based catalysts comprised in the membrane-electrode assembly to be increased. Particularly when using reformates derived from hydrocarbons, significant amounts of carbon monoxide are comprised in the reformer gas and these usually have to be removed by means of a complicated gas treatment or gas purification. The opportunity of increasing the operating temperature enables significantly higher concentrations of carbon monoxide impurities to be tolerated in the long term.
The use of polymer electrolyte membranes based on polyazole polymers enables, firstly, the complicated gas treatment or gas purification to be partly dispensed with and, secondly, the catalyst loading in the membrane-electrode assembly to be reduced. Both are indispensible prerequisites for mass use of PEM fuel cells, since otherwise the costs of a PEM fuel cell system are too high.
J. S. Wainright et al., J. Electrochem. Soc., Vol. 142, No. 7, July 1995, L121-L123, relates to polybenzimidazole films which are doped with phosphoric acid and are potential polymer electrolytes for use in hydrogen/air and direct methanol fuel cells. The electrolytes have a low permeability for methanol vapor, as a result of which the negative effects on the methanol crossover which are usually observed in direct methanol fuel cells can be reduced.
DE 101 176 87 A1 relates to proton-conducting polymer membranes which are based on polyazoles and have a high specific conductivity, in particular at operating temperatures above 100° C., and make do without additional fuel gas humidification. The proton-conducting polymer membranes according to DE 101 176 87 A1 can be obtained by a process comprising the following steps:                A) reaction of one or more aromatic tetramino compounds with one or more aromatic dicarboxylic acids or esters thereof in the melt at temperatures of 350° C.,        B) dissolution of the solid prepolymers obtained as per step A) in polyphosphoric acid,        C) heating of the solution which can be obtained as per step B) under inert gas to temperatures of 300° C. to form the dissolved polyazole polymer,        D) formation of a membrane using a solution of the polyazole polymers as per step C) on a support and        E) treatment of the membrane formed in step D) until it is self-supporting.        
DE 10 2006 036019 A1 relates to a membrane-electrode assembly comprising at least two electrochemically active electrodes which are separated by at least one polymer electrolyte membrane, with the polymer electrolyte membrane having reinforcing elements which penetrate at least partly through the polymer electrolyte membrane. The membrane-electrode assembly is preferably obtained by a process in which    (i) a polymer electrolyte membrane is shaped in the presence of the reinforcing element,    (ii) the membrane and the electrodes are assembled in the desired order.
The membrane-electrode assembly is particularly suitable for use in fuel cells.
The abovementioned polymer membranes based on polyazoles are generally operated in the presence of phosphoric acid as electrolyte.
However, at a high phosphoric acid content, the membranes are soft and therefore have only limited mechanical strength. In addition, the mechanical stability decreases with increasing temperature and the solubility of the polymer framework is increased. In the upper region of the typical operating window of a fuel cell (from about 160 to 180° C.), this can lead to durability problems. Furthermore, the polymer electrolyte membranes can be dissolved or flow away at relatively high temperatures under unfavorable operating conditions. The consequence is failure of the membrane-electrode assembly comprising the abovementioned polymer electrolyte membrane.
It is therefore an object of the present invention to reduce the solubility of polymer electrolyte membranes based on polyazoles in the acid used as electrolyte, preferably phosphoric acid, and to improve the mechanical stability of the membrane.
J.-P. Belieres et al., Chem. Commun., 2006, 4799-4801, describe polymer electrolyte membranes in which ionic liquids are used instead of phosphoric acid as electrolytes. However, polymer electrolyte membranes which comprise phosphoric acid as electrolyte but have a reduced solubility in phosphoric acid compared to polymer electrolyte membranes having phosphoric acid as electrolyte which are customarily used are not disclosed in J.-P Belieres et al.